This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-366459 filed on Dec. 24, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a sensor device that includes a function in itself to detect whether it is normally functioning or malfunctioning. The sensor device is used for measuring a physical amount such as pressure, acceleration, yaw rate, or the like.
2. Description of Related Art
A sensor device for measuring various pressures in an automobile vehicle, such as a braking pressure or a fuel pressure, is connected to a control device such as an electronic control unit (ECU) that controls operation of various devices based on outputs of the sensor device. The sensor device is powered by a power source voltage. A signal representing a detected physical amount is amplified and adjusted in the sensor device and then supplied to the control device as a voltage proportional to the measured physical amount.
The sensor device is connected to the control device via a power source line for supplying power to the sensor device from the control device, a ground line, and an output line through which the sensor outputs are fed to the control device. Those connecting lines are usually connected by connectors, couplers, soldering, welding or the like. Accordingly, there is a possibility that such connections become loose or incomplete, thereby increasing contact resistance at junctions. If resistance in the power source line or the ground line increases, a power source voltage supplied to the sensor device decreases. This results in a malfunction of the sensor device, that is, the outputs of the sensor device do not correctly represent the physical amount to be measured.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved sensor device, in which the sensor malfunction due to the contact resistance increase in the connecting lines is automatically detected.
The sensor device of the present invention includes a sensor circuit, a voltage detector and an oscillator. Preferably, those components are all formed on a single semiconductor substrate. The sensor circuit measures a physical amount such as pressure, acceleration, yaw-rate or the like and outputs a sensor signal (a first signal) representing a measured physical amount. The voltage detector detects a voltage actually supplied to the sensor circuit and outputs a detector signal when the voltage supplied to the sensor circuit is lower than a predetermined level. The oscillator generates a signal (a second signal) oscillating between a high level and a low level, triggered by the detector signal fed from the voltage detector. The second signal indicates that the sensor circuit is malfunctioning because the sensor circuit is operating under a voltage lower than a predetermined level.
The first signal, which is a normal sensor signal, and the second signal indicating the malfunction of the sensor device are selectively supplied to a control device connected to the sensor device. The control device controls various devices connected thereto based on the first signal, while it detects the malfunction of the sensor device based on the second signal.
The maximum level of the first signal is set to a level (e.g., 4.5 V) lower than a normal power source voltage level (e.g., 5.0 V), and its minimum level (e.g., 0.5 V) is set to a level higher than a ground level. The high level of the second signal is set to a level higher than the maximum level of the first signal, and the low level is set to a level lower than the minimum level of the first signal.
The control device connected to the sensor device normally receives the first signal (the sensor signal), while it receives the second signal (the oscillating signal indicating the malfunction of the sensor device) when the voltage actually supplied to the sensor device is lower than the predetermined level.
The selection between the first signal and the second signal may be made by a switching circuit formed in the sensor device. Alternatively, a circuit having a low-pass filter that allows only the first signal having a low frequency to pass through and a high-pass filter that allows only the second signal having a high frequency to pass through may be used for selecting the signals.
The second signal is not fixed to one voltage level, but it oscillates between the high level and the low level, and the malfunction of the sensor device is detected when either the high level or the low level signal is detected. Therefore, the malfunction of the sensor device caused either by a voltage decrease in the power supply line or by a voltage increase in the ground line is surely detected without fail.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.